1. Field of the Invention
The present invention relates to an image signal processing apparatus and an image signal processing method for controlling the optical black level of an image signal.
2. Description of the Related Art
The optical black level of an image signal obtained by an image capturing device (imaging device) can vary according to the intensity of the light received by the device. Such being the case, in the case of a CCD (Charge Coupled Device) solid state image capturing device, a light-shielded pixel region, that is, an optical black (OPB) region, is commonly provided along the outer edge of the effective pixel region in the image capture unit so as to perform processing to clamp the reference level representing the optical black level of an image on the basis of an optical black signal (OPB signal) obtained correspondingly to this OPB region. Also, in an output of an AGC (Auto Gain Control) circuit provided at a subsequent stage, the optical black level of the image signal can vary according to the gain set therein. A clamp circuit to control the optical black level is therefore commonly provided in the subsequent stage of the AGC circuit.
During clamp processing of the optical black level in the related art as is shown in FIG. 6, the final reference optical black level is found by clamping the reference optical black level within the clamp range in the vicinity of the low reference side (L side, which is often on the high voltage side of an image signal in voltage) in the clamp region of an analog-to-digital converting circuit in analog processing, and by applying low-pass filter processing to the reference optical black level in the following digital processing. A desired imaging signal level can be obtained by subtracting the reference optical black level from an imaging signal.
The reference optical black level clamped during analog processing, however, may vary due to noise superimposed on the imaging signal. Hence, when the level to be clamped is adjusted to the low reference side in the clamp region of the analog-to-digital converting circuit, as is shown in FIG. 7, noise below the low reference (that is, exceeding the high voltage limit of the clamp region) is clipped by the analog-to-digital converting circuit in the optical black portion, making it impossible to determine a precise optical black level in the digital processing in the subsequent stage.